When drawing using a CAD software we always draw "real" size (i.e. the scale is 1:1 in the computer space). For example if we draw a box with size 20x20x20 cm we will use 20 drawing units for drawing the sides.

Independently of the scale that we will use to print the drawing we will not scale when we draw on the CAD software. This is a major benefit of CAD software! Nobody wants to hold a calculator the whole time and scale while drawing! Drawing in one thing, scaling for printing is a totally different thing.

We set the scale only at the printing layout which is prepare at a later stage when the drawing is completed and we are ready for printing. In all CAD software, the printing layouts are always a distinct part of the software that helps us fit the drawing to the paper. In all CAD software, drawing is one thing and printing is another thing.

Before we start drawing we decide in our mind what will be the value of a unit length in the drawing. One drawing unit can be equal to one meter, one mm, one cm, one kilometer (km).

It is up to us to decide what the units will be depending on the size actual size of our structure or object. For example, if we draw a coin with a diameter of 20 mm it is convenient to use mm (millimeters). Thus we will draw the coin using a circle with a diameter of 20 units. In this case, one drawing unit represents 1 mm.

If we draw the structural drawings for a concrete house it is much more convenient to use meters (m). In that case one drawing unit represents 1 m. Then a square house with layout of 20x20 meter will be drawn using a square with side-length of 20 drawing units.

Some software might have the option the set this relation, i.e. to set what does one drawing unit represents. However this is not that important. What is important is that you decided and follow the representation when you draw.